The block-replicate architecture which has been widely used in permalloy bubble devices requires the use of swap gates. Swap gates interchange bubbles between the write major loop path and the minor loop paths. Swap gates are referred to as true swap gates when the bubble in one path, for example the major loop, is transferred into the vacancy in the minor loop path that is left by the bubble which is being transferred from the minor loop into the major loop path. Although swap gates are most commonly used to interchange bubbles between the write major loop and the minor loops, they may also be employed in other parts of the bubble device wherever data in two separate paths need to be exchanged. All true swap gates used in permalloy-type bubble devices such as the one described by Bullock et al in the Journal of Applied Physics, Vol. 50, pages 2222-4, March 1979, employ conductors to activate the transfer of bubbles. Due to the fundamental difference between permalloy type bubble devices and ion-implanted contiguous-disk bubble devices, the swap gate designs for permalloy bubble devices cannot be implemented in ion-implanted contiguous-disk bubble devices.
An example of a two-way transfer gate that does not employ conductors for ion-implanted contiguous disk bubble devices was described by R. Wolfe et al in Journal of Applied Physics, Vol. 52, page 2377-2379 (March 1981) and is shown in FIG. 1. This arrangement, which includes a major loop, a minor loop and a swap or idler disk element positioned therebetween is a simple bidirectional reverse rotation transfer gate. FIG. 1 illustrates in three steps how one and a half reverse rotations starting at 270.degree. and ending at 90.degree. are used to transfer a bubble from the upper loop part way around an idler disk to a lower loop. Transfer in the opposite direction is very similar with reverse rotation starting at 90.degree. and ending at 270.degree.. When a transfer-out reverse rotation cycle in this device is followed immediately by a transfer-in sequence, the bit that is removed from the minor loop is replaced by a new one from the major loop. This is not a true swap gate, however, since the bit from the major loop is placed in the minor loop one rotating field cycle behind the replacement bit. For example, the bubble 12 from the major loop is one cycle behind the vacant position 14 which the bubble previously residing in the minor loop occupied prior to the swap operation. In a true swap gate arrangement, the incoming bubble 12 would be transferred into position 14. The FIG. 1 structure is also not a swap gate because data must be completely cleared from the minor loops and positioned in the major loop before new data can be entered into the minor loops. The bubble industry that uses swap gates for their permalloy bubble devices have reported that their work on contiguous-disk bubble devices has been handicapped by the unavailability of a swap gate.
A folded minor loop arrangement for the transfer of bubbles in ion-implanted contiguous-disk bubble devices is disclosed in co-pending patent application Ser. No. 316,250 filed Oct. 29, 1981 and assigned to the assignee of the present invention. This bubble device has a major loop, folded minor loops and a transfer element positioned between the major loop and the folded end of the minor loops. In one embodiment the transfer element has a merge portion adjacent and in spaced relation to one portion of the folded end of the minor loop. While this arrangement provides an improved transfer gate from the major loop to the minor loop, it is not a swap gate because bubbles can be transferred in one direction only.